Hydrophilic, swellable polymers

ABSTRACT

The present invention relates to hydrophilic, swellable polymers which consists of radicals of the general formula I ##STR1## wherein R 1  to R 3  are defined as stated in the description, and of radicals of a crosslinking agent which have originated from monomers having at least two olefinically unsaturated double bonds, and which are characterized in that they are in the form of a highly porous, foam-like polyhedral structure, their preparation and their use as absorption agents for water and aqueous solutions.

The present invention relates to hydrophilic, swellable polymers, theirpreparation and their use.

Swellable polymers which absorb aqueous solutions are used for theproduction of tampons, nappies, sanitary towels and other hygienearticles and as water retention agents in horticultural farms.

Known absorption resins of this type include crosslinkedcarboxymethylcellulose, partly crosslinked polyalkylene oxide,hydrolysis products of starch-acrylonitrile graft copolymers and partlycrosslinked polyacrylic acid salts.

All of these known polymers exhibit disadvantages, especially in theabsorption of aqueous electrolyte solution as well as blood and urine.

According to the current prior art, the gel stabilities of the swollenpolymer particles which are achieved at a high absorption capacity aretoo low. Tacky masses form, which impair the absorbency of the productsproduced with these substances.

It is known that the gel stability and the rate of liquid uptake can beincreased by increasing the crosslinking density, but at the same timethis reduces the absorption capacity. This procedure is undesirableinasmuch as the absorption capacity is the most important property ofthe polymer.

The object of the present invention is to provide polymers which absorbaqueous solutions which have a high rate of absorption and at the sametime do not form tacky hydrogel particles of high gel stability in theswollen state.

This object is achieved, surprisingly, by hydrophilic, swellablepolymers which consist, in random distribution, to the extent of 98 to100% by weight of radicals of the general formula ##STR2## wherein

R¹ denotes hydrogen, methyl or ethyl, R² denotes carboxyl, sulphonyl orphosphonyl, which can optionally be esterified by alkanol having 1 to 4carbon atoms; phenyl; sulphonylphenyl; pyrrolidonyl; pyridyl;imidazolyl; a group of the formula ##STR3## wherein R⁴ represents thesulphonyl or the phosphonyl group; cyano; chlorine; the --CONH₂ group;(C₁ -C₄)-alkanoyloxy; or a group of the formula ##STR4## wherein R⁵ andR⁶ independently of one another denote hydrogen, methyl or ethyl oroptionally also together represent trimethylene; and

R³ denotes hydrogen, methyl, ethyl or carboxyl, it also being possiblefor acid groups to be present in salt form, and to the extent of 0-2% byweight of radicals of a crosslinking agent which have originated frommonomers having at least two olefinically unsaturated double bonds, andare characterized in that they are in the form of a highly porous,foam-like polyhedral structure.

The highly porous, foam-like polyhedral structure preferably has anaverage pore diameter of 0.8 to 1.2 mm.

The preferred density of the polymers according to the invention isbetween 0.01 and 0.05 g/cm³.

Preferred polymers according to the invention consist to the extent of98.5 to 99.7% by weight of radicals of the general formula I and to theextent of 0.3 to 1.5% by weight of crosslinking structures.

In the polymers according to the invention, the radicals of the generalformula I can all have exactly the same structure, but they can alsodiffer from one another in respect of the radicals R¹, R² and R³. In thelatter case, radicals of the general formula I which differ in respectof the meanings of R¹, R² and R³ can alternate in a random manner, butit is also possible for larger polymer sections in which R¹, R² and R³each have only one meaning to follow one another.

In the radicals of the general formula I, R¹ preferably denotes hydrogenor methyl. R² preferably represents carboxyl, sulphonyl or phosphonyl,carboxyl being particularly preferred. R³ preferably denotes hydrogen.

In the radicals of the general formula I, all or some of the acid groupscan be in salt form.

The alkali metal, alkaline earth metal, ammonium and amine salts arepreferred. The sodium and ammonium salts are particularly preferred.

The crosslinking structures mentioned can be derived from all suitablemonomers having at least two olefinically unsaturated double bonds.

Suitable monomers are, for example, compounds which contain at least twoalkenyl groups, for example vinyl or allyl, or at least two alkenoylgroups, for example acrylate or methacrylate.

The crosslinking structures are preferably derived from monomers whichcontain 2, 3 or 4 ethylenically unsaturated double bonds.

The crosslinking structures are particularly preferably derived frommethyl-bisacrylamide or N,N'-dihydroxyethylene-bisacrylamide. Thecrosslinking structures can furthermore also be derived from cationicmonomers, such as, for example, diallyl-dimethyl-ammonium chloride.

The polymers according to the invention can be prepared bypolymerization of 98 to 100% by weight, preferably 98.5 to 99.7% byweight, of a compound of the general formula Ia ##STR5## wherein R¹, R²and R³ are as defined above, or a salt thereof, with 0 to 2% by weight,preferably 0.3 to 1.5% by weight, of a monomer having at least twoolefinically unsaturated double bonds, characterized in that an aqueousmatrix containing a surfactant and the monomers and stabilized by aliquid hydrocarbon phase is employed as the polymerization medium.

The polymerization medium preferably consists of 50 to 99.5% by weightof hydrocarbon, 0.2 to 20% by weight of monomer, 0.1 to 5% by weight ofsurfactant and 0.2 to 25% by weight of water.

The polymerization medium particularly preferably consists of 60 to 99%by weight of hydrocarbon, 0.4 to 16% by weight of monomer, 0.1 to 4% byweight of surfactant and 0.5 to 20% by weight of water.

The polymerization medium is preferably built up by foaming an aqueoussolution containing surfactant and monomer, for which an inert gas, forexample nitrogen, is preferably used, and then adding the liquidhydrocarbon.

During this operation, a quasi-foam structure is formed, the hydrocarbonphase occupying the position of the air in comparison with aconventional foam structure. Analogously to the air/water foamstructure, here also the aqueous phase is the uninterrupted phase, thatis to say continuous phase.

All the customary surfactants of anionic, cationic, and nonionicstructure can be used as the surfactant. The surfactants of thefollowing structures are preferred: ##STR6##

Sodium dodecylsulphate is an especially preferred surfactant.

Saturated and unsaturated aliphatics or aromatics can be used as thehydrocarbons. They can be employed by themselves or as mixtures with oneanother, in any desired mixing ratios.

Preferred hydrocarbons are n-pentane, n-hexane, n-heptene, n-octane,n-nonane, n-decane, n-dodecane, n-tetradecane, n-hexadecane,cyclohexane, cyclooctane, benzene, toluene, kerosine, petrol, lead-freepetrol and diesel oil.

The monomers of the general formula Ia are preferably water-soluble,olefinically unsaturated compounds, acids, such as, for example, acrylicacid, methacrylic acid, vinylsulphonic acid, styrenesulphonic acid,maleic acid, fumaric acid, crotonic acid,2-acrylamido-2-methyl-propanesulphonic acid,2-acrylamido-2-methyl-propanephosphonic acid and vinylphosphonic acidand half-esters thereof, being particularly preferred. Suitablewater-soluble monomers are, for example, also acrylamide,methacrylamide, vinylpyrrolidone, vinylpyridine, vinylimidazoline andN-vinylamides, such as, for example, N-vinyl-N-methyl-acetamide,N-vinyl-formamide and N-vinyl-acetamide.

Water-insoluble monomers can also be added as comonomers within certainlimits. Examples are the alkyl esters of acrylic and methacrylic acid,styrene, vinyl acetate, acrylonitrile and vinyl chloride.

Particularly stable polymers according to the invention are obtained ifthe monomer solution contains crosslinking agents, that is to saypolyolefinically unsaturated compounds which effect three-dimensionalcrosslinking of the polymers. Examples of suitable monomers of this typeare those having at least two alkenyl groups, such as vinyl or allyl, orat least two alkenoyl groups, such as acrylate or methacrylate.Particularly preferred crosslinking agents are methyl-bisacrylamide andN,N'-dihydroxyethylene-bisacrylamide.

The polymers according to the invention can be prepared in the processaccording to the invention by known polymerization reactions.Preferably, a polymerization of water-soluble monomers in aqueoussolution is carried out. The polymerization, which proceeds relativelyrapidly, is accelerated still further by the Norrish-Trommsdorff effect.

The polymerization reaction can be carried out in the temperature rangebetween 0° C. and 130° C., preferably between 10° C. and 100° C., eitherunder normal pressure or under increased pressure. As is customary, thepolymerization can also be carried out in an inert gas atmosphere,preferably under nitrogen.

High-energy electromagnetic radiation or the customary chemicalpolymerization initiators can be used to initiate the polymerizationreaction, for example organic peroxides, such as benzoyl peroxide,tert.-butyl hydroperoxide, methyl ethyl ketoneperoxide, cumenehydroperoxide, azo compounds, such as azo-di-iso-butyro-nitrile, andinorganic peroxy compounds, such as (NH₄)₂ S₂ O₈ or K₂ S₂ O₈ or H₂ O₂,if appropriate in combination with reducing agents, such as sodiumbisulphite, and iron(II) sulphate or redox systems which contain as thereducing component an aliphatic and aromatic sulphinic acid, such asbenzenesulphinic acid and toluenesulphinic acid or derivatives of theseacids, such as, for example, Mannich adducts of sulphinic acid,aldehydes and amino compounds, such as are described in German PatentSpecification 1,301,566. As a rule, 0.03 to 2 g of the polymerizationinitiator are employed per 100 g of total monomers.

After removal of the solvent, hydrocarbon and surfactant, the processaccording to the invention gives highly absorbent, porous polymerstructures which are outstandingly suitable as absorption agents forwater and aqueous solutions, so that they can advantageously be employedas water retention agents in horticultural farms, as filtrationauxiliaries and in particular as absorbent components in hygienearticles, such as nappies, tampons and sanitary towels.

EXAMPLE 1

A solution of 2 g of acrylic acid, 2 g of sodium dodecylsulphate and 0.1g of N,N'-(1,2-dihydroxyethylene)-bisacrylamide in 4.9 g of distilledwater is prepared and initially introduced into the reaction vesseltogether with 40 g of heptane.

To remove the oxygen in the solution, nitrogen is passed through thesystem with the aid of a glass capillary over a period of five minutes.The vessel is now closed with a stopper and the mobile two-phase systemis converted into a gelatinous state by shaking movements. The gelformation is complete when all the heptane has been incorporated intothe gel phase. 0.018 g of potassium peroxodisulphate in 0.882 g ofwater, and 200 μl of 10% strength ascorbic acid solution are now addedand shaken into the gel substance. A start to the polymerization isachieved within 60 minutes in this manner. The start of thepolymerization is clearly recognizable from an increase in temperatureof about 15° C. The reaction is complete about 10 to 15 minutes afterthe start of polymerization.

The polymer substance is now transferred to a dish and treated for about8 hours with a solution of 1.4 g of NaHCO₃ in an amount of water whichis just sufficient to cover the polymer completely. During this period,the size of the cells filled with hydrocarbon increases from about 20-40μm in diameter to about 0.8 to 1.2 mm. Liquid nitrogen is now pouredover the very soft and unstable product and the product is freeze-driedfor 30 minutes in a microwave oven (alternatively, this can also becarried out in a freeze-dryer or rotary evaporator. Time required inthis case about 8 hours).

The product formed is distinguished by a sponge-like structural patternpenetrated by an open, three-dimensional system of channels. The porewidth is 0.8 to 1.2 mm. The density is in the range from 0.01 to 0.5g/cm³.

The following tabular Examples 2 to 36 are prepared analogously toExample 1. In the table, the quantity data denote per cent by weight,based on the total amount used in the reaction.

The following abbreviations are used:

AA: acrylic acid

AMP: 2-acrylamido-2-methyl-propanesulphonic acid

AAM: acrylamide

MA: methylacrylate

TAE: tetraallyloxyethane

DHEBA: N,N'-dihydroxyethylene-bisacrylamide

MBA: methyl-bisacrylamide

KPS: K₂ S₂ O₈

H₂ O₂ : hydrogen peroxide

APS: (NH₄)₂ S₂ O₈

                                      TABLE                                       __________________________________________________________________________                                 Surfac-                                                                           Mono-                                                                             Comon-                                                                             Cross-                                                                              Initi-                                      Cross-         tant                                                                              mer omer linking                                                                             ator                                                                              H.sub.2 O                                                                         Hydrocarbon                Mon-                                                                              Comon-                                                                             linking   Hydro-                                                                             % by                                                                              % by                                                                              % by agent %                                                                             % by                                                                              % by                                                                              % by                  Example                                                                            omer                                                                              omer Agent                                                                              Initiator                                                                          carbon                                                                             weight                                                                            weight                                                                            weight                                                                             by weight                                                                           weight                                                                            weight                                                                            weight                __________________________________________________________________________     2   AA       TAE  KPS  pentane                                                                            1.5 9        0.45  0.09                                                                              18.96                                                                             70                     3   AA       TAE  KPS  hexane                                                                             1   6        0.3   0.06                                                                              12.64                                                                             80                     4   AA       TAE  KPS  heptane                                                                            0.5 3        0.15  0.03                                                                              6.05                                                                              90                     5   AA       TAE  KPS  octane                                                                             0.25                                                                              1.5      0.075 0.015                                                                             3.16                                                                              95                     6   AA       TAE  KPS  diesel oil                                                                         0.1 0.6      0.03  0.006                                                                             1.26                                                                              98                     7   AA       DHEBA                                                                              APS  heptane                                                                            1.5 6        0.45  0.09                                                                              21.96                                                                             70                     8   AA       DHEBA                                                                              APS  decane                                                                             1   4        0.3   0.06                                                                              14.64                                                                             80                     9   AA       DHEBA                                                                              APS  pentane                                                                            0.5 2        0.15  0.03                                                                              7.05                                                                              90                    10   AA       DHEBA                                                                              APS  hexane                                                                             0.25                                                                              1        0.075 0.015                                                                             3.66                                                                              95                    11   AA       DHEBA                                                                              APS  nonane                                                                             0.1 0.4      0.03  0.006                                                                             1.464                                                                             98                    12   AAM      DHEBA                                                                              KPS  pentane                                                                            1.5 12       0.45  0.09                                                                              15.96                                                                             70                    13   AAM      DHEBA                                                                              KPS  hexane                                                                             1   8        0.3   0.06                                                                              10.64                                                                             80                    14   AAM      DHEBA                                                                              KPS  heptane                                                                            0.5 4        0.15  0.03                                                                              5.05                                                                              90                    15   AAM      DHEBA                                                                              KPS  dodecane                                                                           0.25                                                                              2        0.075 0.015                                                                             2.66                                                                              95                    16   AAM      DHEBA                                                                              KPS  octane                                                                             0.1 0.8      0.03  0.006                                                                             1.064                                                                             98                    17   AMP      MBA  KPS  pentane                                                                            6   9        0.3   0.06                                                                              14.64                                                                             70                    18   AMP      MBA  KPS  hexane                                                                             4   6        0.2   0.04                                                                              9.76                                                                              80                    19   AMP      MBA  KPS  heptane                                                                            2   3        0.1   0.02                                                                              4.88                                                                              90                    20   AMP      MBA  KPS  decane                                                                             1   1.5      0.05  0.01                                                                              2.44                                                                              95                    21   AMP      MBA  KPS  diesel oil                                                                         0.4 0.6      0.02  0.004                                                                             0.976                                                                             98                    22   AA  MA   DHEBA                                                                              KPS  pentane                                                                            6   3   3    0.3   0.06                                                                              17.64                                                                             70                    23   AA  MA   DHEBA                                                                              KPS  hexane                                                                             4   1   3    0.2   0.04                                                                              11.76                                                                             80                    24   AA  MA   DHEBA                                                                              KPS  heptane                                                                            2   1.5 0.5  0.1   0.02                                                                              5.88                                                                              90                    25   AA  MA   DHEBA                                                                              KPS  octane                                                                             1   0.5 0.5  0.05  0.01                                                                              2.94                                                                              95                    26   AA  MA   DHEBA                                                                              KPS  nonane                                                                             0.4 0.1 0.3  0.02  0.004                                                                             1.176                                                                             98                    27   AAM      TAE  H.sub.2 O.sub.2                                                                    pentane                                                                            6   6        0.3   0.06                                                                              17.64                                                                             70                    28   AAM      TAE  H.sub.2 O.sub.2                                                                    hexane                                                                             4   4        0.2   0.04                                                                              11.76                                                                             80                    29   AAM      TAE  H.sub.2 O.sub.2                                                                    heptane                                                                            2   2        0.1   0.02                                                                              5.88                                                                              90                    30   AAM      TAE  H.sub.2 O.sub.2                                                                    octane                                                                             1   1        0.05  0.01                                                                              2.94                                                                              95                    31   AAM      TAE  H.sub.2 O.sub.2                                                                    nonane                                                                             0.4 0.4      0.02  0.004                                                                             1.176                                                                             98                    32   AA       DHEBA                                                                              APS  decane                                                                             6   12       0.3   0.6 11.64                                                                             70                    33   AA       DHEBA                                                                              APS  cyclo-                                                                             4   8        0.2   0.04                                                                              7.76                                                                              80                                            hexane                                                34   AA       DHEBA                                                                              APS  benzene                                                                            2   4        0.1   0.02                                                                              3.88                                                                              90                    35   AA       DHEBA                                                                              APS  toluene                                                                            1   2        0.05  0.01                                                                              1.94                                                                              95                    36   AA       DHEBA                                                                              APS  diesel oil                                                                         0.4 0.8      0.02  0.004                                                                             0.776                                                                             98                    __________________________________________________________________________

What is claimed is:
 1. Hydrophilic, swellable polymer which consists inrandom distribution, to the extent of 98 to 100% by weight of radicalsof the general formula I ##STR7## wherein R¹ denotes hydrogen, methyl orethyl,R² denotes carboxyl, or carboxyl which is esterified by alkanolhaving 1 to 4 carbon atoms; and R³ denotes hydrogen, methyl, ethyl orcarboxyl, it also being possible for acid groups to be present in saltform, and to the extent of 0-2% by weight of radicals of a crosslinkingagent which having originated from monomers having at lest twoolefinically unsaturated double bonds, characterized in that it is inthe form of a highly porous, foam-like polyhedral structure.
 2. Polymeraccording to claim 1, characterized in that the polyhedral structure hasan average pore diameter of 0.8 to 1.2 mm.
 3. Polymer according to claim1, characterized in that it consists to the extent of 98.5 to 99.7% byweight of radicals of the general formula I and to the extent of 0.3 to1.5% by weight of crosslinking structures
 4. Polymer according to claim1, characterized in that in the general formula I, R¹ denotes hydrogenor methyl, R² denotes carboxyl, and R³ denotes hydrogen.
 5. Polymeraccording to claim 1, characterized in that the radicals of acrosslinking agent are derived from methyl-bisacrylamide orN,N'-dihydroxyethylene-bisacrylamide.
 6. Polymer according to claim 1,wehrein the density of the polymer is between 0.01 and 0.05 g/cm³.